An attempt was made to assess the ''medium distance'' of sampling sites from vehicular emission sources. Two parameters, the ''site ratio'' and the ''traffic-equivalent distance'', were calculated from the time-weighted average (TWA) concentrations of two selected volatile air pollutants, benzene and toluene. The concept of the model was based on the considerable difference between the rate constants of benzene and toluene with OH radicals. It was shown that these parameters are more reliable indicators of the distances of emission sources than the absolute TWA concentrations which are greatly influenced by the intensities of the emission sources, the location of the sampling site, and the duration of sampling.
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